________with his parents that he is able to tell who they are from their steps.     [ ]

A.  So familiar her is                            

B.  So familiar is he    

C. So he is familiar                              

D. So is he familiar

在一项队列研究中，计算出某研究因素的RR值的95％的可信区间为0.2～0.8，那么该研究因素可能为（）。

A.危险因素

B.保护因素

C.有害因素

D.无关因素

E.以上均不是